As produced, carbon blacks are powdery materials with bulk densities ranging from about 0.02 to 0.1 g/cc and are termed fluffy blacks. Because of their low densities and large surface areas, the fluffy products are cohesive, have very poor conveying properties and are very dusty. They are, however, dispersible. Because of their poor handling properties, advantage of their excellent dispersibilities cannot be taken in many applications. For example, fluffy blacks cannot be fed in a controlled manner to standard dispersing devices, such as Banbury mixers, twin screw extruders or the like.
To improve their handling properties, the fluffy products are densified. For a given grade of black, handling properties tend to improve with increasing degrees of densification. Dispersibility, on the other hand, is progressively degraded as the extent of densification is increased. Thus there is a tradeoff between improvement in bulk handling and degradation in dispersibility. For this reason, the extent and means employed to densify the fluffy products depend on their intended uses.
The industry, in general, uses three basic methods to attain densification. These, in order of providing increased levels of densification, are: agitation or vacuum treatment of the fluffy product, dry pelletization and wet pelletization. Since the performance of carbon black in many applications depends on the degree of dispersion attained, the acceptable extent of densification achieved depends on the user's dispersion equipment and, especially, on the shearing stresses generated. The process of agitation or vacuum treatment yields a powder which cannot be bulk handled and is supplied only in a bagged form. Nevertheless, because this form of the product is much more dispersible than its more dense counterparts, it is used in applications where easy dispersion is mandatory.
Dry pelletization is conducted in rotating drums. Industrial drums have diameters of 6 to 10 feet and lengths of 20 to 40 feet which are rotated at 5 to 20 RPM. The fluffy product is fed continuously to one end of the drum. Tumbling of the dry black results in the formation of small round pellets. The process of pellet formation is facilitated by the use of seed pellets, which, typically, consist of part of the product pellets which are recycled to the feeding end of the drum. Generally, the products formed in dry drums have relatively low densities and, hence, are relatively weak and have low attrition resistances. As a consequence, conveying can cause pellet breakdown which leads to a degradation in their bulk handling properties. Many methods are available for enhancing pellet strengths. These methods include addition of small quantities of oil and binder.
Wet pelletizing is conducted in pin pelletizers. Such units consist of a cylinder which is 0.4 to 1.5 m in diameter and up to 3 to 4 m long. Along the axis of the unit is a rotating shaft which is fitted with a multitude of pins, typically, arranged in the form of helices with the pins extending almost to the cylinder wall. The rotational velocity of the shaft depends on the diameter of the unit and the intensity of pelletizing desired. Rotation speeds can range from 300 up to 1500 RPM. The fluffy black and water are continuously added to the unit. The combination of capillary forces generated by the water in the black-water mixture and the mechanical action of the pins results in the formation of spherical, wet pellets with diameters, mostly, in the range of 0.25 to 3 mm. The water/black ratio required in the pelletizing operation depends on the structure of the black and, in many cases, is in the range of 1:1. The wet pellets exiting the pelletizer are then dried in rotary driers. Because of the high moisture contents in the pellets, drying represents a costly unit operation.
Despite the reduction in pellet dispersibilities and the attendant costs of drying, pin pelletizing is extensively practiced because it yields more dense, attrition-resistant pellets than the dry process. Further, binders, such as lignosulfonates, sugars or molasses as well as additives such as polyoxyethylene nonionic surfactants, substituted polyethylene glycol, etc. can be easily added to the pelletizing water. These serve to strengthen or, when surfactants are used, strengthen and improve the dispersibilities of the dried pellets.
The industry has also attempted to improve the tradeoff between enhanced pellet strength and degradation in dispersibility by providing moisture-free, oil-containing pellets. A maximum of 8 weight % oil in the black can be tolerated without changing its hazard classification. Oil can be easily incorporated in a black by means of the dry pelletization process. At oil levels much above 15 weight %, the pellets have been characterized as being "too soft and mushy to handle well in bulk".
Aqueous emulsions of oil have been used to form oil-containing pellets in various mixing devices. It can be expected that, in most cases, drying will result in loss of oil by steam distillation and necessitate additional processing steps.
Pin pelletizing can also be accomplished with pure oil in place of water/oil emulsions. In such instances water removal by drying is not required so that loss of the oil will no longer occur. However, for pellet formation, the oil contents of pellets will be substantially larger than 8 weight %, necessitating a change in their hazard classification.
Another approach taken to improve the tradeoff between enhanced pellet strength and reduced dispersibility has been to pelletize carbon black with aqueous media containing latexes which are compatible with rubber. The resulting pellet compositions, after drying, were found to have superior handling and dispersibility properties in rubber applications. Other workers, as described in U.S. Pat. No. 4,569,834, have pelletized carbon black with aqueous dispersions of waxy polyalkalenes, such as polyethylene waxes, and also found that the dried pellets exhibited improved handling and dispersibility properties. In these cases, however, pelletizing is effected in the presence of water so that drying, a costly unit operation, has to be employed. Also, the additives must be either available as or formed into aqueous emulsions or dispersions. Further, they must be thermally stable at the maximum drying temperatures attained in the rotary dryers used in the industry. These factors limit the range of materials which can be used in the pelletizing operation. A further limitation is that the additive must be compatible with the medium in which it is used. Nevertheless, such pellet compositions, formed by pin pelletizing carbon black with aqueous emulsions and dispersions of various compounds, have utility.
Other workers have developed an improved agglomeration process wherein an aqueous slurry of carbon black is mixed with an oil having a softening temperature in excess of about 100.degree. C.
Mednikov et al. used up to 5 weight % of molten high density polyethylene, having a melt temperature of 125.degree. to 135.degree. C., to strengthen dry process pellets. This disclosure is found in Mednikov, M. M., V. M. Osipov, I. G. Zaidman, V. I. Ivanovskii, S. V. Oreklov and A. I. Ryabinkov, "The Use of PE in Dry Pelletization of Carbon Black," International Polymer Science and Technology, Vol. 9, No. 1, T/37 (1982). The viscosity of such polymers are high with typical values exceeding 20 Pa.multidot.s at a shear rate of 10 s.sup.-1 at 190.degree. C. These workers introduced solid polyethylene into air-borne fluffy carbon black having a temperature of 180.degree. to 210.degree. C. It was claimed that the polymer melted and was then adsorbed onto the surface of the black. The black was subsequently dry pelletized at an unspecified temperature to give pellets which had mass pellet strengths which were 2 to 5 kg higher than those, about 8 kg, formed in the absence of the polyethylene. While the process of Mednikov et al. gives some enhancement in pellet strength, the gain in strength attained was relatively small. Further exemplification of the foregoing disclosure appears in East German Patent No. 133,442 covering this technology. It should be noted that in Example 1 of this patent it is stated that the polyethylene used had a molecular weight of 2600. This is inconsistent with the stated molecular weight range of 15000 to 150000 said to be useful for the practice of the invention. Furthermore, as stated in the patent, the molten polyethylene serves as a site for forming agglomerates (by adhesion of the black to its surface). This indicates that the molten polymer is viscous. Otherwise, the polyethylene, being present as a minor constituent (less than 5 weight %) would have migrated into the intra-aggregate pores. For this reason, it would appear that the stated molecular weight for the polyethylene in Example 1 is not a correctly stated value. This contention is supported by the data in the present application where it is established that no strength enhancement is attained when using a low viscosity melt at the levels utilized in East German Patent 133,442.
Another approach to form dispersible pellets with good bulk handling properties was taken by Wallcott in U.S. Pat. No. 3,429,958. Wallcott pelletized carbon black with a molten paraffin wax in a pin mixer. The resultant cooled pellets, containing about 50 weight % wax, were free-flowing and found to be more dispersible than conventional wet process pellets in ink media. In his work, Wallcott used HAF (DBP=102 cc/100 g), SAF (DBP=113 cc/100 g) and ISAF (DBP=114 cc/100 g) blacks as examples of furnace blacks. Wallcott stated that, for furnace blacks, the weight ratio of carbon black to wax must be in the order of about 50:50 and claimed that the ratio must lie between 50:50 to 30:70. Thus, the process requires the use of relatively high wax levels.
The process developed by Walcott represents a considerable advance in the art. However, for many applications the wax levels employed by Wallcot are excessive. In many applications there is a preferred wax level (e.g., for lubricity, mold release, gloss, improved mar resistance, etc.) above which product performance is degraded. The preferred wax level, often, is smaller than the black loading. Accordingly, use of pellets containing 50% or more wax to attain the desired black loading will, inevitably, result in the addition of more than the desired wax level leading to a degradation in performance and increased costs. For certain applications it is preferred that the wax level of the pellets always be less than 48% by weight. Moreover, as will be further described, the process of pin pelletization becomes progressively more difficult as the level of liquid wax is increased and, for many blacks, becomes impossible at a 48% wax level.
The difficulties encountered both in handling carbon black pellets and in pellet dispersion have resulted in the establishment of businesses which produce concentrated dispersions of carbon blacks in aqueous and non-aqueous media (often referred to as masterbatches or concentrates). The production of masterbatches in thermoplastic polymers is of special importance. In this application, pelletized black is dispersed in a heated, viscous thermoplastic material such as polyethylene, polypropylene, acrylonitrile-butadiene-styrene copolymer, ethylene vinyl acetate, etc. Dispersion is effected in standard dispersing equipment such as Banbury mixers or twin screw extruders or the like. For production of acceptable masterbatches, the formation of good quality dispersions is of critical importance. After the dispersion process is complete, the masterbatch is, for example, extruded and then sliced into pellets for shipment.
The loading of black in the pellets is, as implied by the name "concentrate", quite high and will depend on the structure of the black. Carbon black consists of aggregates composed of partially coalesced primary particles. The spaces between the primary particles form the intra-aggregate void or pore volume. Structure has been shown to be related to the average number of primary particles per aggregate. This is found in Medalia, A. I., "Morphology of Aggregates: 6. Effective Volume of Aggregates of Carbon Black From Electron Microscopy: Application to Vehicle Absorption and to Die Swell in Filled Rubber," J. Colloid and Interface Science, 32, 115 (1970). A measure of this volume can be found by evaluating the n-dibutyl phthalate absorption, DBP, of the black by means of the ASTM D 2414 procedure. This value represents a measure of the volume of liquid required to fill the intra- and inter-aggregate pores of the dispersed black at the capillary state. The carbon black aggregates, in the black-DBP mix at the capillary state, are taken to be close to their maximum packing fraction.
For economic reasons, high loadings of black in a masterbatch or concentrate are preferred. However, for rapid incorporation during let-down, the viscosity of the concentrate should not be very different from that of the medium in which it is being dispersed. Concentrate viscosity increases with pigment loading and approaches a high value as its solids content approaches that required for the pigment to attain its maximum packing fraction. Accordingly, to obtain acceptable viscosities, the black loading in a masterbatch will be less than that at which it attains its maximum packing fraction and, hence, contains little or no air. In other words, the black loading is less than that required to achieve the capillary state.
In contrast to conventional masterbatches, the pellets of this invention are formed at black loadings which exceed the capillary state so that they contain air. As a consequence, they can appear to be much more viscous than conventional masterbatches. The effect of air on viscosity, however, is mitigated in pressure rheometers because the high pressure employed can reduce the volume of voids between the black aggregates. Medalia and Sawyer have demonstrated that carbon blacks are highly compressible. This is discussed in Medalia, A. I., and R. L. Sawyer, "Compressibility of Carbon Black, Proc. Fifth Carbon Conference, 1961," Pergammon Press, N.Y., 1963, p. 563. The criterion that the pellets of this invention are formed on the "dry" side of the capillary state (i.e., they contain air) in agglomeration devices with molten organic compounds in the absence of water may be used to distinguish them from conventional masterbatches, such as those described in the literature and which, typically, are formed on the "wet" side of the capillary state (i.e., the masterbatch pellets are essentially void-free). The expressions "dry" side and "wet" side of the capillary state are used solely to indicate whether pellets comprising a black-organic compound mixture contain air or are air-free, respectively.
The maximum black content of a conventional masterbatch will depend on the maximum acceptable viscosity. For reasons already discussed, the volume of polymer in the masterbatch is substantially larger than that required to attain the capillary state as measured by the black DBP value. For the same masterbatch viscosity and for blacks with comparable surface areas, the loading that Can be achieved increases with decreasing black DBP.
Black dispersibility decreases as black surface area increases and/or its DBP decreases. Because of difficulties encountered in their dispersion (and depending on the application), blacks with low DBP values and very high surface areas are rarely used to form masterbatches. For example, for applications where jetness or UV protection is needed, the black must have a minimum surface area. To form acceptable concentrates with a black having a high surface area, a high DBP product may often be used in concentrate formation. Thus, practical considerations dictate that in masterbatch formation a compromise be struck between black loading and dispersion quality. For this reason, blacks with the lowest attainable DBP values are rarely used in the production of black masterbatches.
In spite of their costs, the market for black concentrates or masterbatches is substantial because the resulting products are dust-free, easily conveyed and much more easily dispersed in compatible thermoplastic media than conventionally pelletized blacks. Surprisingly, we have found that carbon blacks pelletized with a molten organic compound or a mixture of organic compounds which are solid at ambient temperatures, can be used in place of concentrates without significant loss in performance.